Bug 43272 suggested "Established users" as a replacement for Autoconfirmed.

Note that newbie is everyone not autoconfirmed (or with an equivalent access permission), defined as such by core mediawiki (User::isNewbie).

It is also mentioned in:
'group-autoconfirmed.css' => '/* CSS placed here will affect autoconfirmed users only */', # only translate this message to other languages if you have to change it
'group-autoconfirmed.js' => '/* Any JavaScript here will be loaded for autoconfirmed users only */', # only translate this message to other languages if you have to change it

Although those entries are more technical.

It's definitely jargon, and probably somewhat unfriendly to new users. A term like "established" could be better, but we need to make clear that it still has a special technical meaning (not just a subjective view of who is established)

Also, we would need to decide about the confirmed group. Do we distinguish "established" and "autoestablished", or perhaps "manuallyestablished" and "established"?

This is a personal preference but... I find manuallyestablished an ugly word :)

I agree. :) It wasn't meant to be an ideal term, but mainly food for thought.

However, "confirmed" (the current equivalent to "manuallyestablished") isn't used that much in practice, so the term doesn't need to be perfect.

"Confirmed" isn't used much, but perhaps should be used more; in my experience, very few know what it is about. Clearer terms would surely be a good thing.

Perhaps the User Experience team might have some ideas.

More food for thought: automatically accepted, automatically assumed non-newbie, assumed to be not a newbie, having basic experience, out-of-quarantine.